Scaling Relationship for NO2 Pollution and Urban Population Size: A

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Scaling relationship for NO2 pollution and urban population size: A satellite perspective Lok Lamsal, Randall Martin, David D. Parrish, and Nickolay A. Krotkov Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es400744g • Publication Date (Web): 13 Jun 2013 Downloaded from http://pubs.acs.org on June 18, 2013

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Environmental Science & Technology

Scaling relationship for NO2 pollution and urban population size: A satellite perspective

L. N. Lamsal1, 2, R. V. Martin3, 4, D. D. Parrish5, N. A. Krotkov2

1

Goddard Earth Sciences Technology & Research, Universities Space Research

Association, Columbia, MD 21044-3432, USA 2

NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

3

Department of Physics and Atmospheric Science, Dalhousie University, NS, Canada

4

Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA

5

Chemical Sciences Division, Earth System Research Laboratory,

National Oceanic and Atmospheric Administration, Boulder, CO 80305, USA

Correspondence to: L. N. Lamsal ([email protected])

Abstract Concern is growing about the effects of urbanization on air pollution and health. Nitrogen dioxide (NO2) released primarily from combustion processes such as traffic is a short-lived atmospheric pollutant that serves as an air quality indicator, and is itself a health concern. We derive a global distribution of ground-level NO2 concentrations from tropospheric NO2 columns retrieved from the OMI satellite instrument. Local scaling factors from a three-dimensional chemistry-transport model (GEOS-Chem) are used to relate the OMI NO2 columns to ground-level

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concentrations. The OMI-derived surface NO2 data are significantly correlated (r=0.69) with in situ surface measurements. We examine how the OMI-derived ground-level NO2 concentrations, OMI-derived NO2 columns, and bottom-up NOx emission inventories relate to urban population. Emission hot spots such as power plants are excluded to focus on urban relationships. The correlation of surface NO2 with population is significant for the three countries and one continent examined here: United States (r=0.71), Europe (r=0.67), China (r=0.69) and India (r=0.59). Urban NO2 pollution, likelike other urban properties, is a power law scaling function of the population size: NO2 concentration increases proportional to population raised to an exponent. The value of the exponent varies by region from 0.36 for India to 0.66 for China reflecting regional differences in industrial development and per capita emissions. It has been generally established that energy efficiency increases and therefore per capita NOx emissions decrease with urban population; here we show how outdoor ambient NO2 concentrations depend upon urban population in different global regions.


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1 Introduction More than half of the world’s population now lives in cities, with the number of urban dwellers and percentage of total population living in urban areas predicted to rise rapidly in coming decades.1 Cities require and consume large quantities of energy and materials that can generate severe ambient air pollution, which is responsible for millions of annual excess deaths worldwide2-3, so concern is growing about the effects of global urbanization on health due to air quality degradation.4-6 Differences exist in per capita emissions of air pollutants between rural and urban areas as well as between developing and developed countries due to differences in energy consumption rates and energy production infrastructure.

However, only very limited quantitative

information exists regarding how ambient pollutant concentrations relate to urban population within a given country, and how that relationship varies between countries. Our goal in this paper is to use a global data set of ambient air pollutant concentrations to investigate this relationship. To our knowledge, this is the first systematic, observation-based, quantitative investigation of the dependence of ambient air pollutant concentration on urban population on a global scale.

Nitrogen dioxide (NO2) is an air quality indicator of combustion emissions that is associated with respiratory mortality and morbidity.7-10 Furthermore, nitrogen oxides (NOx = NO2 + NO) contribute to the formation of fine particulate matter (PM2.5) and ozone (O3), both of which are harmful to human health and the environment.11-13 The World Health Organization (WHO) provides air quality guidelines for major air pollutants, including NO2, for protecting public health from adverse effects of environmental pollutants.14 Our focus in this paper is on the global data set of NO2 concentrations that is available from satellite measurements.15



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Short-lived air pollutants of anthropogenic origin such as NO2 are primarily confined close to combustion source regions and, therefore, can be used to investigate how air pollution exposure and pollutant emissions relate to urban population. A reliable quantitative analysis of the global relationships between NO2 and population requires a globally consistent dataset.

Satellite remote sensing of NO2 has advanced

dramatically over the past two decades with finer spatial resolution.16 Furthermore, NO2 retrieval algorithms have matured considerably and now offer high consistency between different algorithms.17-18 Satellite remote sensing is emerging as a powerful new information source to connectinform population exposure to air pollution.19-20 Here we assess population exposure to NO2 air pollution from the globally consistent dataset derived from satellite observations.

Diverse urban characteristics such as infrastructure, energy consumption, employment, and innovation are related to population size.21 The theory of the urban system has an analogue in biology.22-23 Urban air pollution produced as a result of resource consumption (e.g. fuel combustion) may follow a scaling relationship

with

population, as do many diverse urban properties. This study takes advantage of the satellite NO2 observations that offer a global dataset of consistent quality for quantitative investigation of the relationship between urban pollution and population. We further assess the relationship using additional independent information contained in bottom-up NOx emission inventories.

2 Methods

2.1 Satellite Observations The OMI instrument offers early afternoon (local time 1300-1430) NO2 column abundance at spatial resolution of up to 13 × 24 km2 at nadir with daily global


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coverage.22 We use the OMI standard tropospheric NO2 column data for 2005 product (standard product version 2.1, collection 3) available from the NASA GES DISC (http://disc.sci.gsfc.nasa.gov/Aura/data-holdings/OMI/omno2_v003.shtml). Tropospheric NO2 columns are derived with a three-step approach: (1) retrieval of NO2 abundance along the average viewing path (slant column) with a Differential Optical Absorption Spectroscopy (DOAS) algorithm23 in the 405-465 nm wavelength range, (2) separation of stratospheric and tropospheric NO2 components17, and (3) computation of an air mass factor by integrating the NO2 relative vertical distribution (shape factors) of NO2 weighted by altitude and cloud dependent scattering weight factors to convert the slant columns into NO2 vertical columns.24 Individual clear-sky (cloud radiance fraction < 0.5) observations were allocated by area-weights into 0.25˚ × 0.25˚ grids. The estimated errors in the tropospheric NO2 columns under clear‐sky conditions and typical urban concentrations (1×1015 molec. cm-2) are ∼30%.17,25 Pixels at swath edges with ground pixel sizearea >20km × 63km1209km2 were excluded to reduce spatial smearing of the measured tropospheric NO2.

2.2 GEOS-Chem Model We use the GEOS-Chem three-dimensional model of tropospheric chemistry26, version 8-03-01 (www.geos-chem.org), to simulate the relationship between OMI retrievals of the tropospheric NO2 columns and ground-level concentrations. We employ GEOS-Chem nested simulations27-29 at 0.5˚ × 0.667˚ over North America (10ᵒN-70ᵒN, 40ᵒW-140ᵒW), Europe (30ᵒN-70ᵒN, 30ᵒW-50ᵒE), and East/Southeast Asia (11ᵒN-55ᵒN, 70ᵒE-150ᵒE) and a global simulation at 2.0˚ × 2.5˚ elsewhere. Boundary conditions of the nested region are provided by athe global simulation at 2.0˚ × 2.5˚. The GEOS-Chem simulation is driven by assimilated meteorological data available from the Goddard Earth Observing System (GEOS-5) at the NASA Global Modeling and Assimilation Office. The model includes a detailed simulation of



tropospheric ozone-NOx-hydrocarbon chemistry as well as of aerosols and their precursors.26,30

The global anthropogenic emissions in this GEOS-Chem simulation are from EDGAR 3.2FT200031 at 1ox1o resolution for 2000, which are scaled to 2005 following van Donkelaar et al.32 The global inventory is overwritten by regional inventories: The U.S. EPA National Emissions Inventory (NEI) for 2005 over the United States, the CAC inventory (http://www.ec.gc.ca/inrp-npri) for 2005 over Canada, the BRAVO inventory33 for 1999 over Mexico, the EMEP inventory for 2005 over Europe, the inventory from Zhang et al.34 for 2006 over East Asia. NOx emissions from soils, lightning, biomass burning, and aircraft are as described in Lamsal et al.35-36 Anthropogenic emissions represent > 75% of total NOx emissions for North America, Europe, and East Asia.35

The GEOS-Chem simulation of NOx has been evaluated extensively with in situ and satellite observations and generally agrees to within 30% of measured NOx. 37-40 We conduct a simulation for the year 2005 and sample the model output between 13:00 and 15:00 local time for analysis of the OMI data.

2.3 Estimation of Ground-Level NO2 from OMI We estimate ground-level NO2 concentrations from OMI to examine the relationship of NO2 with population. We follow the approach of Lamsal et al.15 that combines simulated local NO2 vertical profile with satellite observations of tropospheric NO2 column, but with an improved representation of the spatial variation of NO2 concentrations in the boundary layer. We use the GEOS-Chem NO2 vertical profile coincident with the OMI observations.


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The ground-level NO2 mixing ratio S is estimated from OMI tropospheric NO2 column Ω following Lamsal et al.35:

 S  

= ν +

F Ω G  SG (ν +1) B × ×Ω ΩG  ΩG

………………………. (1)

Here the subscript “G” represents GEOS-Chem. The symbol ν is the ratio of the local OMI NO2 column Ω to mean OMI NO2 column over a GEOS-Chem grid Ω from B

OMI. The spatial variation of boundary layer NO2 columns Ω is calculated from ν F

assuming negligible horizontal variation in free-tropospheric NO2 columns Ω due to the longer NOx lifetime in the free troposphere. The calculated ground-level NO2 calculated represents the mixing ratio at the lowest vertical layer (~100 m) of the model. Results from equation (1) differ from the earlier approach of Lamsal et al. 15, 35 by up to 10% due to explicit dependence of the ratio of the free-tropospheric to boundary layer concentrations.

NO2 concentrations derived from previous retrievals of OMI NO2 (version 1.0) exhibited significantstrong daily (r=0.3-0.8, mean r = 0.53) and seasonal-spatial (r=0.73) correlations with in situ surface measurements at 307 sites over North America after correction for artifacts in the surface measurements.15 We find that both the daily (r=0.3-0.96, mean r = 0.69) and seasonal-spatial (r=0.78) correlations are further improved with increase for surface concentrations derived from the improved OMI NO2 product (version 2.1) used here.



2.4 Population Data Population densities at a resolution of 0.25˚ × 0.25˚ for 2005 were obtained from the Center for International Earth Science Information Network (CIESIN), Columbia University.41 Coordinates of each city were used to identify a 0.25˚ × 0.25˚ grid for calculation of population density and NO2 concentration. The center of population of the city was identified by comparing the population density of the selected grid with that of the 8 surrounding grid boxes. The population of the urban area was calculated from population density and area of the 9 grid boxes with the grid of highest population density designated as urban center. This approach provides an urban domain consisting of an urban core and surrounding areas from where its population likely commutes, but disregards the political or geographic boundaries.

We selected 238 US, 757 European, 245 Chinese, and 265 Indian cities with population greater than 50,000. The selection excludes urban domains that contain large power plants (identified as CO2 annual emissions >18 Tg for the year 2007 using the data available from CARMA (www.carma.org)). Since emission inventories are more accurate in more developed countries, this exclusion may be more accurate for the United States and Europe, than for India and China.42 The population over the 0.75˚x0.75˚ wide urban domain ranges from 0.08-14.8 million for Europe and the United States, 0.1-15.3 million for China, and 0.5-23.5 million for India.

3 Results and Discussion Figure 1 shows the OMI-derived annual mean surface NO2 concentrations in the afternoon (OMI-overpass time) for 2005 over regions of major anthropogenic sources: North America, Europe, China, and India. These regions contain two-thirds of total global anthropogenic NOx emissions.35 The annual mean NO2 concentrations vary spatially by more than 2 orders of magnitude with < 0.1 ppbv in rural areas and >10


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ppbv in urban areas. The early afternoon annual average surface NO2 concentrations shown here are lower (by about a factor of two) than either nighttime or wintertime values.15 The spatial variation in NO2 reflects the spatial distribution of combustion sources such as traffic. Numerous individual cities can be clearly identified in mean NO2 concentrations. Contours of population density overlaid onto surface NO2 concentrations in Figure 1 demonstrate that most large scale NO2 enhancements are over regions of high population density.

Urban NO2 levels for the selected urban domains exhibit considerable regional differences with lowest concentrations of 0.2-1.2 ppbv in Indian cities and highest concentrations of 0.3-8.0 ppbv in Chinese cities. Concentrations for Indian cities reflect low NOx emissions due to low per capita fuel consumption rates.43 Annual mean NO2 mixing ratios vary from 0.2 to 6.9 ppbv for the US and European cities, with an overall mean of 1.2 ppbv.

Figure 2 shows the log-log relationship between NO2 mixing ratios and the total population of urban areas for the four regions, after excluding cities with large power plants. The approximate linear relationship for each region is notable, suggesting that the dependence of urban NO2 pollution on population follows a power law scaling with population as has been found for many other urban indicators21:

Υ

β

= C × P . ………………………………………. (2)

Here, Y represents the average NO2 mixing ratio for an urban area with population P. The exponent, β, contains information on the relationship between ambient NO2 concentration and population in urban areas. The exponents for the four regions are equal to the slopes of the linear least-square lines shown in Figure 2. The intercept of



each linear fit in Fig. 2 is equal to lnC for the region, where C is the average NO2 mixing ratio for an urban area with a population of 1 million.

The insets of Figure 2 contain correlation coefficients and the β values from the relationships between NO2 and population for the four selected regions. NO2 mixing ratios are significantly correlated (r=0.59-0.71) with urban populations in each of the four regions. The β values are in the range 0.36-0.66, with an overall mean of 0.48. A value of β

Scaling Relationship for NO2 Pollution and Urban Population Size: A

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